Have a nice summer

A stock phrase signed by high school students in a yearbook of a classmate they have no previous memory of ever encountering.

Mentioned during the first season of Buffy the Vampire Slayer in the episode 'Out of Mind, Out of Sight', when a student of Sunnydale High who had turned invisible was revealed to have a yearbook filled with the generic sentiment.

Harmony: That girl just gave me her yearbook to sign but I have no clue who she is!
Cordelia: Just write "Have a nice summer" in it and hand it back.